Here's why:

* Composition: Oceanic crust is primarily composed of denser basalt and gabbro, while continental crust is composed of lighter granitic rocks.

* Formation: Oceanic crust is formed at mid-ocean ridges through volcanic activity, while continental crust is formed through a combination of volcanic activity and tectonic collisions. This difference in formation leads to variations in thickness.

* Age: Oceanic crust is constantly being renewed at mid-ocean ridges, making it generally younger and thinner than continental crust, which is much older and thicker.

Typical thicknesses:

* Oceanic crust: 5-10 kilometers (3-6 miles)

* Continental crust: 30-70 kilometers (19-43 miles)

Therefore, continental crust is significantly thicker than oceanic crust.